1. Field of the Invention
The present invention relates, in general, to driver circuits, and, more particularly, to systems and methods for compensating for electrical time constant of an inductive load with switched voltage-mode power drivers and other circuits in which an important system variable is controlling the current level in an inductive load.
2. Relevant Background
Switched power driver circuits are widely used to generate power suitable for driving loads such as motors. Switched power drivers turn on and off repetitively to supply regulated current in an efficient manner (i.e., with minimal switching loss). Switched power driver circuits are associated with driver circuits that control, for example, the magnitude (by means of the duty cycle of the on and off cycles), so as to supply a desired amount of power to a load. In a typical application, a power driver circuit is controlled by a system processor, often implemented as a microcontroller IC, that generates commands to the driver circuit. The driver circuit essentially turns on and off in response to the received commands. When on, the driver circuit supplies current to the load, and when off, the driver circuit cuts off current supply to the load.
Motor loads are inductive, making their behavior (e.g., speed and direction) related to the magnitude and polarity of the current supplied. The inductive nature of the load causes a lag or delay, referred to as an electrical time constant, in the current level change in the load caused by a change of voltage across the load terminals. Essentially, voltage across the terminals of an inductor can change instantaneously, however, current cannot change instantaneously. For precision motor control applications such as required in disk drive storage systems, this time constant delay can lead to positioning errors and slower performance.
For example, a typical application involves control of a voice coil motor (VCM) that controls position of a head assembly with respect to a disk surface in a hard disk drive. Slower transient variations in the load current caused by the electrical time constant translate into position errors. The disk drive system must compensate for the errors by waiting for the transient condition to subside before writing or reading data from the desired location.
Head position control is implemented by a servo control system. Early servo control systems for low density drives used open loop positioning using stepper motor technology. However, at higher densities closed loop solutions are required. Current disk drives, for example, obtain head position information directly from data contained on the disk surface. A track number, in the form of encoded binary data, is recorded at various locations about the disk surface and uniquely identifies each recording track on the disk. Servo position, in the form of sinusoidal burst signals staggered in position between adjacent tracks can be used to determine the position of the head with respect to a track centerline. The track number and servo burst are used to compute a position error signal (PES), which is fed into the electromechanical servo position system.
In operation, a disk drive controller generates a command to move the head to a particular location, and the command is translated into voltage signals applied to the VCM. The voltage signals, often called drive signals, may be linear or switched-mode. Switched mode drivers are also known as pulse width modulation (PWM), phase shift modulation (PSM) and other names. Switched-mode drivers can be implemented as current-mode (i.e., use a current minor loop) or as voltage-mode drivers. These switched mode drivers have the advantage of reducing the power dissipation to the driver devices and therefore allow smaller devices and packages. In a current mode driver, a feedback loop is typically used to compare the actual current applied to the VCM to the requested current. The applied signal is compared to the command to compute a current error signal (CES). The CES is used to modify the command value, so that the head eventually moves to the desired location and reduces the PES value.
In the case of the driver for a VCM in a disk drive, a typical circuit topology for the feedback loop is one that is termed a xe2x80x9cCurrent Minor Loopxe2x80x9d (CML). This refers to a feedback circuit that generates a signal that is proportional to or indicative of the current magnitude in the load. The xe2x80x9cminorxe2x80x9d loop term indicates that there is localized feedback in the driver circuit in contrast to the xe2x80x9cmajorxe2x80x9d loop that controls the head position. The current in the load is sensed by an external current sense resistor, for example, that is coupled in series with the load. This resistor is typically a high precision power resistor that is relatively expensive. The inductive nature of the load causes a lag or delay in the current level change in the load due to a change of voltage across the load terminals. With a CML, the voltage across the load is increased at a greater rate of change than the command by means of the feedback loop. This faster rate of change is controlled by a compensator stage within the loop and has limits imposed by stability criteria.
The voltage across the resistor is brought into a control IC through an additional pin. The measured voltage is compared to the presently requested command value and a corrected command value is applied to the power drivers to obtain a load current that is proportional to command value. This extra IC pin required to port in the resistor voltage is not desirable in highly integrated circuits due to an increase in package cost. Moreover, the extra IC pin displaces other functionality that could be implemented using the pin. As higher levels of circuit integration are desired and the reduction of external components and pins is desirable due to cost and circuit area constraints, the CML topology is undesirable.
One feedback topology that eliminates the need for the external current sense resistor and additional pin is a voltage-mode driver. Voltage mode power drivers refer to a class of control circuits that regulate the output voltage as opposed to output current. Voltage mode drivers are desirable because they require fewer device I/O pins to monitor and regulate the supplied power. Presently available switched voltage-mode power drivers do not sense the current in the load and therefore have no means to correct for lag in the current response caused by time constant related delays.
While voltage and current in an inductive load are related, a voltage-mode driver regulates current in the inductive load indirectly by monitoring and regulating the applied voltage. In this case, the average voltage output of the driver is proportional to the command value. When the load is purely resistive, the output average voltage value can be sensed and used to correct for changes caused by load impedance in a feedback method similar to the CML, but without the external current sense resistor and extra pin. However, as a result of inductive load impedance, the resulting current does not change instantaneously for a change in terminal voltage. Similarly, in capacitive loads the terminal voltage is not indicative of the load current.
The current change in an inductive load is exponential and the rate of this change is described by its electrical time constant (xcfx84). This value is calculated by dividing the inductance measured in Henries by the resistance measured in Ohms. The resulting time constant is in the units of seconds. The current, as a function of time, is I(t)=Iss*(1xe2x88x92e(xe2x88x92t/xcfx84)) where Iss is the final steady state current, t is the time variable in seconds, and xcfx84 is the electrical time constant in seconds. In the frequency domain this same characteristic of an inductive load can be described in the admittance function, I(t)/V(t), of the load. In that case the admittance decreases at the idealized corner frequency of 1/(2xcfx80xcfx84). For example, an actuator in a 2.5xe2x80x3 disk drive could have a xcfx84 of 19.2 xcexcS. The resulting lag corner frequency of the admittance function would be at 8.3 KHz. In the time domain, for a step in voltage, the current increases exponentially to 63% of the final Iss value in the time equal to one xe2x80x9ctauxe2x80x9d. It is desirable to increase this current rate of change and the admittance corner frequency in order to reduce phase lag in the tracking control system and to reduce time delays in the seek or velocity control system of the disk drive. This affects the performance of the device in which the driver is being used. The command to the power driver is updated at a rate that is matched to the feedback information rate of the system. In a disk drive the feedback information rate is conventionally the servo sector rate. However, this rate is too slow to effectively compensate for the electrical time constant at the rate that the commands are provided by the main loop processor. A need exists for a system and method that allows the driver to correct at a much faster rate.
Briefly stated, the present invention involves a power driver for driving a signal on a load using a voltage-mode driver. A system processor generates commands indicating a programmed drive signal desired for the voltage-mode driver. A lead compensator, in the case of an inductive load, determines a compensation factor to compensate for an admittance function of the load. The compensated commands are coupled to the voltage-mode driver, such that the voltage-mode driver generates a voltage output based upon the compensated command.
In another aspect, the present invention involves a method of driving an electrical load, wherein the load has a characteristic admittance function. A voltage-mode driver is coupled generating a drive signal to the electrical load. Commands indicating a programmed voltage output desired from the voltage-mode driver are generated. At least one time lead compensation coefficient is determined canceling an electrical time delay associated with the load. The commands are modified using the at least one time lead compensation coefficient to produce a compensated command. The compensated command is supplied to the voltage-mode driver. A voltage output from the voltage-mode driver is generated based upon the compensated command.